A gypsum plasterboard or wallboard typically comprises a core layer of gypsum (calcium sulphate dihydrate) sandwiched between two outer layers of a liner, such as a paper liner.
Such boards are generally produced in a continuous process using calcined gypsum as a starting material. Calcined gypsum (also known as stucco) is calcium sulphate dihydrate that has been heated to drive off at least some of the bound water molecules, and therefore comprises mainly calcium sulphate hemihydrate and in certain cases also some calcium sulphate anhydrite.
The stucco is fed into a mixer with water and additives to produce a stucco slurry that is deposited onto a continuously advancing paper liner moving on a conveyor. The slurry is allowed to spread over the advancing paper liner before a second liner is laid over the slurry to provide a continuous band of wallboard preform having a sandwich structure. The preform may be subjected to a shaping process to meet the desired thickness. As the band of stucco slurry is moved forward by the conveyor belt, the stucco reacts with the water in the slurry to form calcium sulphate dihydrate, and begins to set. When the stucco reaches a point in the production line that the setting process is sufficiently advanced, segments of the stucco band are cut off and sent for drying.
A parameter of critical importance in the production of gypsum wallboard is the water gauge of the stucco slurry (that is, the water-to-stucco ratio by mass). The water content of the slurry is considerably in excess of that required for the stucco to form calcium sulphate dihydrate. The additional water is required to provide the stucco slurry with sufficient fluidity to allow the slurry to spread evenly when it is deposited on the wallboard liner.
The plasterboard panels produced in this way are commonly used to provide partitions within buildings. Their advantages for this application include the fact that they are light and quick to install. However, in certain cases, plasterboard panels may have the drawback that they are not strong enough to support fixtures (e.g. sinks, televisions, radiators, fire extinguishers, shelves and any other item that requires attachment to the panel). In such cases, the weight of the fixture may cause the fixing means (e.g. screws) to be pulled out of the panel, such that the fixture falls away from the partition.
Typically, this problem has been addressed by providing plywood sheets to increase the fixing strength of the panel. In this case, the plywood sheet is fixed to the supporting frame of the panel, and the plasterboard subsequently affixed to the plywood sheet, such that the plywood sheet is provided on the side of the panel opposite to that on which the fixture is to be located. The plywood sheet may provide increased strength for retaining one or more fixing means (e.g. screws) employed to secure the fixture to the panel. Typically, the plywood sheet is positioned within the partition framework, and the plasterboard then fixed to the plywood, so that it lies outside the partition framework.
As an alternative, metal support means may be provided. These may comprise fixing plates, channels, straps, or metal fasteners. As is the case for plywood sheets, the metal support means are generally positioned on the side of the panel opposite that to which the fixture is to be secured, and act to receive and secure fixing means, e.g. fixing screws, that are used to attach the fixture to the panel.
Both these arrangements have the disadvantage that they require additional installation actions to be carried out on-site to secure the panels and the additional supporting components. Moreover, when metal support means are used, a plurality of such support means may be needed to support the full set of fixing means required to secure the fixture to the panel. Thus, the installation process may be time-consuming and expensive.
Furthermore, the addition of metal support means or plywood sheets increases the weight and thickness of the partition, and/or results in a reduction in cavity wall space. In general, the plywood itself must be cut to size on site, thus increasing the time required for installation and possibly leading to the release of dust and potentially harmful components.
It has been found that by including relatively high levels of polymer additives (such as starch), as well as fibres (e.g. glass fibres) in the plasterboard, significant improvements in fixing strength may be achieved.
However, it is thought that the presence of polymer (particularly starch) and fibre additives in the stucco slurry leads to reduced slurry fluidity, unless the water gauge of the slurry is increased.